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Stand alone air purifiers work very well as long as they are sized properly. You should have a minimum of 4 air changes per hour. They work best in a one room environment - for example in the bedroom of an allergic and/or asthmatic person.

Three brands we have tested and found to be very effective are IQ Air, Amaircare and Austin Air.

noise

A major consideration for portable ones is the noise level. It doesn't matter how efficient it is if you won't turn it on. Stay clear of the small ones, they're too noisy. You want the biggest one you can afford that you will run at the lowest speed. Even IQ Air ones are noisy.

Also, an air purifier can be completely ineffective if there is much infiltration of dusty air, like trying to collect water in a sieve. For example, poor windows, or average windows on a windy day, may make running an air purifier pointless. The only way to be sure if an air purifier is effective is to own a particle counter like the Dylos. You may have to seal the windows in a room (tape, caulking, etc...)

pm
It is not just the efficiency of the filter that makes for effective air cleaning. Air changes per hour have a much bigger role than one assumes.

For example, I have been testing a Vornado Air Purifier. It is very quiet. In fact, I can hear the Dylos particle counter vacuum over the air purifier. At the normal speed it is producing 386 cfm. This means that it is getting 17 air changes per hour in my office. We are using MERV 11 filters in the unit.

The Dylos particle count on the 5 micron size particles is zero. The particle count on the 1 micron size particles is 2,800. Particle counts at 0.3 microns are 282,000. In an office down the hall the 5 micron particle size count is 800. The 1 micron size count is 23,200. And the 0.3 micron size particle count is 840,000.

Even though the one pass counts through a HEPA filter are much better, the cumulative effect of multiple passes through a MERV 11 filter give comparable results in room particle counts.

pm
It is not just the efficiency of the filter that makes for effective air cleaning. Air changes per hour have a much bigger role than one assumes.

For example, I have been testing a Vornado Air Purifier. It is very quiet. In fact, I can hear the Dylos particle counter vacuum over the air purifier. At the normal speed it is producing 386 cfm. This means that it is getting 17 air changes per hour in my office. We are using MERV 11 filters in the unit.

The Dylos particle count on the 5 micron size particles is zero. The particle count on the 1 micron size particles is 2,800. Particle counts at 0.3 microns are 282,000. In an office down the hall the 5 micron particle size count is 800. The 1 micron size count is 23,200. And the 0.3 micron size particle count is 840,000.

Even though the one pass counts through a HEPA filter are much better, the cumulative effect of multiple passes through a MERV 11 filter give comparable results in room particle counts.

That's a lot of air delivered quietly, I'm impressed. However, the air changes is only one factor. Your filter is in a contest with the infiltration of new dust. So if (air changes) X efficiency@size <= 100x (new dust infiltration rate in air changes) does not hold, the filter is not good enough.
Even at these very high "air changes", per your own measurements, the MERV 11 is ineffective on 0.3 uM particles. A reduction of 3-4x is not enough to matter, and is completely negligible for strong allergies. Typically you want 100x fewer particles or better to be able to get below the "detection level" of the allergic person. In practice a MERV 11 filter only works at that level on coarse particles like pollen.

For example, with moderate activity in my house the particle counts at 0.5 and 2.5 microns are respectively 10 and 2 (1000 and 200 particles per cubic foot), without a portable filter running in the room. A frenetic ruffling of papers and moving about will increase it to about 60 and 15. While just sitting and typing it will go down to 8 and 0 (800 and <50). This is what I need for my allergies to quiet down. I believe that I'd be uncomfortable in your office (and possibly cold from the wind, although I imagine it may feel nice in Texas ). I suppose that means I need to be in a class 1000 clean room for my allergies to go away (according to a graph I saw, class 1000 means 1000 particles or fewer per cubic feet at 0.5 microns http://www.ee.byu.edu/cleanroom/particlecount.phtml).

Can you make a *livable* class 1000 clean room out of an ordinary office with just MERV 11 filters? If so I'll be impressed.

math

the removal of dust is
dC/dt = -C (E F/V)
where
C is the concentration of particles (particles/cubic feet)
E is the efficiency of the filter @ size
F is the cfm of the device
V is the volume of the room

At equilibrium this is equal to the infiltration rate:
dCi/dt = D (L/V)
where D is the dusty (outside) concentration of particles (particles/cubic feet)
L is the flow in cfm through openings
V is the volume of the room

C (E F/V) = D (L/V)
we want C= D/100 so
E F/V = 100 D L/V
or in air changes,
A = 100 I/E
where A is the number of air changes of the device and I is the number of air changes due to infiltration. E is the efficiency of the filter @ size.
So for a MERV 11 filter with an efficiency at 0.5 microns of about 1% (?) and an infiltration rate of 0.1 (tight building), the number of air changes needed would be 100*0.1/0.01 = 1000 air changes.

So, you would need 50 of those vornado devices in your office before my allergies would be OK, considering a 0.5 uM allergen.